1. Field of the Invention
This invention relates to adhesive compositions, and more particularly, it relates to the use of fully hydrolyzed vinyl alcohol copolymer as protective colloid in vinyl acetate-ethylene copolymer aqueous emulsions.
2. Description of the Prior Art
It has long been the object of research to provide stable vinyl acetate copolymer emulsions capable of yielding adhesive bonds which exhibit optimum water resistance and high strength. U.S. Pat. No. 3,563,851 discloses that one of the more frequently used modifiers for vinyl acetate homo- or copolymer emulsions is polyvinyl alcohol which aids in increasing the viscosity and improving the machineability and moistenability of the adhesive. However, it is taught therein that such emulsions containing polyvinyl alcohol are inherently poor with respect to their water resistance. According to this patent the addition of certain acidic, metal salt curing agents to the aqueous emulsions of copolymers of vinyl acetate, which emulsions also contain polyvinyl alcohol as a protective colloid, yields adhesive bonds with outstanding water resistance. The vinyl acetate copolymers dealt with in this patent are those of alkyl esters of acrylic and methacrylic acid, substituted or unsubstituted mono- or dialkyl esters of alpha,beta-unsaturated dicarboxylic acid, alpha,beta-unsaturated carboxylic acid, vinyl halides, vinylidene halides, and amides of alpha,beta-unsaturated carboxylic acids.
U.S. Pat. No. 3,708,388 discloses adhesive compositions consisting of aqueous vinyl acetate-ethylene copolymer emulsions containing about 5 to 40 percent by weight ethylene and a solids content of about 45 to 60 percent wherein the vinyl acetate and ethylene monomers are copolymerized in the presence of a polyvinyl alcohol, hydroxyethyl cellulose, casein, hydroxyethyl starch, carboxymethyl cellulose, gum arabic, etc. protective colloid. The use of polyvinyl alcohol acetylated up to 50 percent is disclosed and specific examples using a combination of two polyvinyl alcohols, each of which has a degree of hydrolysis of 87-89 mole percent, was given. This patent does not contain any teaching concerning the use of any special grade of polyvinyl alcohol in order to obtain improved water resistance. In teaching the suitability of a long list of protective colloids, they are presented as equivalents.
U.S. Pat. No. 3,827,996 discloses vinyl ester polymer dispersions which have good stability, water resistance, and viscosity index obtained by copolymerizing vinyl ester and ethylene in an aqueous medium in the presence of partially hydrolyzed polyvinyl alcohol having an average vinyl acetate content of about 5 to 7 mole percent consisting essentially of polylvinyl alcohol molecules containing no less than about 0.5 mole percent nor more than about 16 mole percent vinyl acetate. This patent does not disclose the use of vinyl alcohol/methyl methacrylate copolymers nor does it suggest that good water resistance could be obtained thereby.
U.S. Pat. No. 3,906,135 discloses an adhesive composition that, when cured, provides a bond having excellent water resistance. According to the disclosure vinyl acetate polymer/polyvinyl alcohol aqueous emulsion adhesives can yield cured bonds with substantially improved water resistance by including in the adhesive a monocarboxylic acid which can esterify polyvinyl alcohol. Curing of the adhesive composition is recommended at 225.degree. F. to 375.degree. F. The polyvinyl alcohol employed is 75 to 100 percent hydrolyzed.
U.S. Pat. No. 3,689,469 relates to copolymers of vinyl alcohol and methyl methacrylate having a residual vinyl acetate content of no more than 0.5 mole percent (i.e., at least 99.5 mole percent hydrolyzed) consisting of 94 to 98 percent vinyl alcohol and 6 to 2 percent by weight methyl methacrylate. The copolymer is disclosed to be useful where many commercial grades of polyvinyl alcohol are used. A particularly preferred use is as a textile yarn warp-sizing agent. There is no disclosure or even the faintest suggestion herein that the use of this vinyl alcohol copolymer as a protective colloid in the preparation of vinyl acetate-ethylene emulsions would result in an adhesive having excellent water resistance.